1. Field of the Invention
The present invention relates to a solenoid valve drive system that drives a solenoid valve at high speed by applying high voltage at the start of solenoid valve driving.
2. Background Art
High-speed driving of a solenoid valve is commonly effected by the well-known method of applying a high voltage that is higher than the power supply voltage to the solenoid valve for a prescribed time period at the initial stage of solenoid valve driving. As taught by Japanese Public Disclosure Nos. Sho 63-34387 and Hei 6-26589, for example, the high voltage used for this purpose is obtained by stepping up DC voltage for driving a control circuit with a step-up chopper. In this type of solenoid valve drive apparatus, a switching device for turning the current supplied to the solenoid valve on and off is used, for example, to on/off control current supplied to the solenoid valve from the step-up chopper and to control the supply of hold current to the solenoid valve after high-voltage driving. Owing to safety considerations, a switching device of this type is generally installed on the high side of the solenoid valve. Usable switching devices include the pnp transistor, p-channel field-effect transistor (FET) and n-channel FET. In view of the need to minimize energy loss in the switching device, however, an n-channel FET is actually used since it has the smallest loss.
Use of an n-channel FET on the high side of the solenoid valve generally requires use of a gate driving voltage of about 30 V, which is higher than the power supply voltage. Since this gate driving voltage is higher than the power supply voltage but lower than the aforesaid high voltage, it has to be separately generated in the drive apparatus. The gate driving voltage can be obtained by, for example, generating it by use of a separately provided step-up circuit used exclusively for this purpose or securing it by use of a voltage step-down circuit comprising a resistor and a voltage-regulation diode installed on the output side of the voltage step-up circuit for generating the high voltage. The former arrangement increases cost owing to the additional circuitry required. The latter arrangement is simple in circuitry but increases energy loss because the high-voltage energy stored in the capacitor of the step-up circuit for generating the high voltage is constantly consumed by the step-down circuit. A voltage of around 30 V is also frequently required for other purposes.